High purity metallurgical silicon and method for preparing same

ABSTRACT

The invention concerns a silicon designed in particular for making solar cells containing a total of impurities ranging between 100 and 400 ppm, a boron content ranging between 0.5 and 3 ppm, a phosphorus/boron content ratio ranging between 1 and 3, and a content of metal elements ranging between 30 and 300 ppm. The invention also concerns a method for making such a silicon from an oxygen- or chorine-refined metallurgical silicon containing at least 500 ppm of metal elements, and comprising: refusion under neutral atmosphere of the refined silicon, in an electric furnace equipped with a hot crucible; transferring the molten silicon, to provide a plasma refining, in an electric furnace equipped with a hot crucible; plasma refining with as plasma-forming gas a mixture of argon and of at least a gas belonging the group consisting of chlorine, fluorine, HCI and HF; casting under controlled atmosphere in an ingot mould wherein is produced segregated solidification.

FIELD OF THE INVENTION

The invention relates to a high purity metallurgical silicon withdifferent applications including the manufacture of panels forconversion of light energy, and particularly solar energy, intoelectrical energy. The invention also relates to the process for makingthis material called photovoltaic silicon.

STATE OF THE ART

There are many applications of high purity silicon and there areparticular specifications for each. The purity required for thephotovoltaic application may extend over a relatively wide range, sincethe energy efficiency and the behaviour of solar cells when aging dependon the quality of the high purity silicon used, which leaves solar panelmanufacturers possibilities for making choices in terms of thequality/price ratio.

For a long time, declassified products derived from the production ofelectronic silicon have formed the main source of photovoltaic qualitysilicon, but this source is insufficient to supply the increasing marketdemand, so that other silicon sources are necessary such asmetallurgical silicon produced by carbothermal reduction of silica in anelectric arc furnace, and used mainly as a raw material in chemistry andas an aluminium alloying element. However, the specifications for thequality of metallurgical silicon used for synthesis of chlorosilanes, araw material used in making silicones, are very different from thespecifications for photovoltaic silicon, and it can only be envisagedfor this application if the refining done is very pure. The cost of thisrefining increases very quickly with the degree of purity, the cost ofelectronic quality silicon is thus of the order of 30 times the cost ofmetallurgical silicon.

The specifications required by the photovoltaic application depend onthe qualities requested for cells; the specification for a siliconnecessary to obtain the best performances are boron <3 ppm, phosphorus<10 ppm, total metallic impurities <300 ppm, and preferably <150 ppm.

The cost of refining to obtain this degree of purity remains high and isnot very competitive for a photovoltaic application. One of thetechniques for refining liquid silicon is plasma refining that has beendeveloping gradually and that provides a means of lowering the boron andphosphorus contents to values of a few ppm.

Patent FR 2585690 by Rhône-Poulenc Spécialités Chimiques describes a twostep refining process comprising plasma melting using a hydrogen—argonmix as the plasma-forming gas, followed by plasma refining with ahydrogen—argon—oxygen mix as a gas. This technique has severaldisadvantages:

-   -   firstly it uses hydrogen at high temperature, which in an        industrial use of the process can cause hydrogen leaks and        therefore safety problems that are difficult to solve,    -   the problem of making plasma melting, for which productivity is        low,    -   the problem of using oxygen that generates slag during the        treatment, the said slag forms a barrier between the liquid        silicon being refined and the plasma constituents, which slows        the refining rate. Furthermore, this slag gradually collects on        the edges of the crucible to form a solid slag ring at the        surface of the liquid, which means that slagging off will be        necessary later on. Repeated slagging off during operations        damages the crucibles and weakens them and reduces their life.

Patent EP 0.459.421 (Kawasaki Steel) describes refining of silicon byplasma in a siliceous crucible, or a crucible coated with a siliceousrefractory lining, using an inert gas as a plasma-forming gas to which0.1 to 10% of water vapour is added, and optionally silica powder in aproportion less than 1 kg of silica per Nm³ of gas.

As in the previous case, this operating method encourages the formationof an oxide film on the silicon surface, which has the consequence ofslowing the refining rate. The patent even specifies that an oxygencontent of 0.05% in the molten silicon should not be exceeded.

Furthermore, the fact of using a blown plasma or transferred plasmatorch introduces impurities in the silicon, since wear of the torchcathode due to volatilisation of the metal of which it is composedcontributes to polluting the plasma formed which in turn pollutes thesilicon.

CNRS Patent FR 2.772.741 describes refining of liquid silicon with agaseous chlorine—hydrogen—water vapour mix that has the samedisadvantages as in the previous case, particularly a very low solidsilicon melting rate, and also has the disadvantage of working with acold crucible that introduces large heat losses and very high energyconsumptions of the order of 50 000 kWh/t to 100 000 kWh/t, whereas 8100kWh/t is sufficient for manufacturing liquid silicon by carboreductionof silica, and melting of solid silicon only requires 900 kWh/t.Furthermore, the cold crucible technique cannot be used to designindustrial sized tools.

Therefore, the purpose of the invention is to obtain silicon derivedfrom metallurgical silicon with a purity acceptable for use as aphotovoltaic material, and an economic process for making this materialon an industrial scale from metallurgical silicon.

PURPOSE OF THE INVENTION

The invention concerns a silicon designed in particular for making solarcells, with a total content of impurities ranging between 100 and 400ppm and preferably between 100 and 250 ppm, a content of metallicelements between 30 and 300 ppm and preferably between 30 and 150 ppm, aboron content ranging between 0.5 and 3 ppm, and preferably between 0.5and 2 ppm, and a phosphorus/boron ratio ranging between 1 and 3.

The invention also concerns a method for making a silicon of thisquality from an oxygen or chlorine refined metallurgical siliconcontaining less than 500 ppm of metal elements and comprising:

-   -   remelting of the solid silicon containing less than 500 ppm of        metallic elements under a neutral atmosphere, in an electric        furnace equipped with a hot crucible,    -   transfer of the molten silicon for plasma refining, in an        electric furnace equipped with a hot crucible,    -   plasma refining with a mixture of argon and at least one gas        from the group consisting of chlorine, fluorine, hydrochloric        acid and hydrofluoric acid, as plasma-forming gas,    -   casting under controlled atmosphere in an ingot mould, in which        segregated solidification takes place.

A metallurgical silicon containing less than 500 ppm of metallicelements is preferably prepared using a first segregated solidificationoperation.

DESCRIPTION OF THE INVENTION

Research carried out by the applicant has showed that under someconditions and for some elements, a degree of purity less than thedegree of purity for electronic quality silicon is sufficient forphotovoltaic silicon, and can give good performances for photovoltaiccells.

Thus, phosphorus appears to be a significantly less disturbing elementthan boron and a phosphorus content of up to 10 ppm can be acceptedwithout excessively degrading the performance of cells. Furthermore,impurities other than group III and V elements in the Mendeleiev'sclassification have a minor importance on cell performances and asilicon containing a total amount of impurities of less than 400 ppm, aboron content of between 0.5 and 3 ppm and a phosphorus content ofbetween 1 and 3 times the boron content gives excellent results,provided that the total of all metallic impurities remains below 300 ppmand preferably below 150 ppm.

A material of this type is very useful if it can be produced at acompetitive cost, in other words very much less expensive thanelectronic quality silicon. The manufacturing process according to theinvention satisfies this need. The basic raw material is metallurgicalsilicon produced industrially by carbothermal reduction of quartz in anelectric arc furnace, and more precisely the “chemical” quality thatwill be used for synthesis of chlorosilanes for manufacturing silicones.This grade is obtained using an oxidising refining of metallurgicalsilicon in the liquid state and provides a means of achievingspecifications for example such as Fe<0.30%, Ca<0.10%, Al<0.30%. Boroncontents are usually between 20 and 50 ppm and phosphorus contents arebetween 30 and 100 ppm. This type of material also contains otherimpurities, particularly titanium, carbon and oxygen, but also vanadium,chromium, manganese, nickel and copper in trace state.

The use of a chlorine refining process like that described by theapplicant in patent EP 0.720.967 already provides a means of achievingoxygen levels of 400 ppm on liquid silicon. Experience has also shownthat this type of refining can achieve carbon levels of the order of 100ppm.

Moreover, access to low iron raw materials, and the development of thecomposite electrodes technology, now enables a significant reduction ofiron contents in metallurgical silicon; moreover, the segregationtechnique during solidification can help to further significantly reduceiron contents in this type of silicon, if it should be necessary.

The next step after this first oxygen or chlorine refining is segregatedsolidification, if necessary, to separate a solid silicon containingless than 500 ppm of metallic elements and concentrate the metallicimpurities in an enriched liquid silicon containing 0.5 to 1% ofmetallic elements. Cooling of the poured mass is controlled to limit thefront advance velocity, that must remain below 2×10⁻⁵ m/s and preferablybelow 10⁻⁵ m/s; and 48 to 52% of the solid silicon is obtained with lessthan 500 ppm of metallic elements.

The next step is remelting of solid silicon containing less than 500 ppmof metallic elements, by batch under a neutral atmosphere such as argon,in an electric furnace, and preferably an induction furnace, in aconventional hot crucible made either of carbon or graphite or siliconcarbide, or with a refractory lining composed of sintered silica. Theelectric generator supplying electrical power to the induction furnaceoperates at frequencies typically between 500 and 5000 Hz, depending onthe crucible diameter. A heel is kept after each pour to facilitaterestarting the next operation.

The molten silicon is then transferred into a second electric furnace,preferably an induction furnace equipped with the same type of cruciblefor the plasma refining. The plasma is obtained by an inductive torchpowered by an electricity source at a frequency of between 100 kHz and 4MHz. The plasma-forming gas used for this operation is composed of a mixof argon and chlorine, fluorine, hydrochloric or hydrofluoric acid, theproportion of argon used being between 5 and 90% and preferably between50 and 70%. Under these conditions, refining of silicon leads to theformation of gaseous compounds which avoids the formation of liquid orsolid slag in the furnace crucible. Start up of the operation can befacilitated by adding a small amount of anhydrous magnesium chloride onthe silicon surface, without disturbing refining.

The next step is pouring into an ingot mould under a controlled inertatmosphere, in which a second segregated solidification operation iscarried out; cooling of the cast mass is controlled so that the frontadvance velocity at this stage of the process can be controlled andremains below 10⁻⁵ m/s and preferably below 5×10⁻⁶ m/s. The proportionof solid silicon obtained with less than 300 ppm of metallic impuritiesis between 80% and 86% and the proportion of remaining liquid siliconenriched with metallic elements is between 14% and 20%.

The liquid silicon is preferably transferred between the remeltingoperation and the plasma refining operation by displacement of theassembly composed of the casing of the induction furnace containing aninduction coil, the crucible, and liquid silicon, from the remeltingstation as far as the plasma treatment station. This assembly is builtso that it can be quickly disconnected from and reconnected to 500Hz-5000 Hz static electricity generators installed on the meltingstation and the plasma treatment station. The plasma treatment stationis also equipped with a fixed inductive plasma torch and its electricgenerator, that is also fixed. This arrangement of the equipment andthis procedure are designed to avoid reladling of liquid silicon.

This complete system is capable of preparing about 48 to 52% of siliconcontaining 0.5 to 1% of metallic impurities, 7 to 10% of siliconcontaining 500 to 1500 ppm of metallic impurities and 40 to 43% of highpurity silicon according to the invention, starting from siliconcontaining 0.25% of iron, 0.10% of calcium, 0.30% of aluminium, 20 to 50ppm of boron and 30 to 100 ppm of phosphorus.

Energy consumption is approximately 7000 kWh/t of high purity silicon,plus about 11000 kWh/t necessary to produce the basic material, giving atotal of the order of 18000 kWh/t for the high purity silicon obtained.

The composition of the high purity silicon obtained by this process isas follows:

-   -   Boron 0.5 to 3 ppm; Phosphorus/Boron ratio between 1 and 3;    -   Total impurities: 100 to 400 ppm    -   Total metallic impurities: 30 to 300 ppm, Fe 10 to 200 ppm,    -   Carbon 20 to 50 ppm; Oxygen 50 to 100 ppm;    -   Calcium 5 to 30 ppm; Aluminium 5 to 30 ppm; Titanium 2 to 20        ppm.

A high purity silicon with a significantly better quality can beobtained by increasing the duration of the plasma treatment:

-   -   Boron 0.5 to 2 ppm; Phosphorus/Boron ratio between 1 and 3;    -   Total impurities: 100 to 250 ppm,    -   Total metallic impurities 30 to 150 ppm, Fe 10 to 20 ppm,    -   Carbon 10 to 30 ppm; Oxygen 20 to 50 ppm;    -   Calcium 5 to 10 ppm; Aluminium 5 to 10 ppm; Titanium 2 to 5 ppm.

EXAMPLES Example 1

A 2 t ladle of liquid silicon was taken from the silicon production froman electric submerged arc furnace, and oxidising refining was carriedout on it; the liquid silicon sampled in the ladle gave the followingICP analysis:

-   -   Iron=0.24%; Calcium=98 ppm; Aluminium=245 ppm; Titanium=240 ppm;        Boron=32 ppm; Phosphorus=19 ppm; Carbon=100 ppm; Oxygen=1200        ppm.

Some of the liquid silicon was poured in an ingot mould made of sinteredsilica equipped with a pouring spout with a capacity of 600 kg ofsilicon; this ingot mould with an area of 2 m² was placed in anelectrically heated reverberatory furnace using graphite bars acting asresistances, heat losses from the furnace taking place mainly throughthe hearth. The furnace power was adjusted to 40 kW to achieve 50%solidification of the silicon in about 1.25 h. After 75 minutes waiting,the liquid remaining in the ingot mould was poured through the spout andproduced a 290 kg ingot.

The mass of solid silicon remaining in the ingot mould was 294 kg andits analysis was: Iron=285 ppm; Calcium=24 ppm; Aluminium=14 ppm;Titanium: =9 ppm; Boron=28 ppm; Phosphorus=10 ppm; Carbon=100 ppm;Oxygen=800 ppm. The operation was repeated several times to obtain asufficient quantity of this quality of silicon.

The silicon obtained was remelted in 200 kg batches under an argonatmosphere in a 250 kW induction furnace operating at 1200 Hz beginningwith a metallurgical liquid silicon heel; the production from the firstthree batches was discarded to be sure that the furnace was correctlyflushed.

The production from the next batches was transferred batch by batch fromthe remelting station to the plasma treatment station for refiningtreatment by completely transferring the assembly composed of the casingof the furnace, the induction coil, the crucible, and liquid silicon.The plasma treatment station is equipped with a generator identical tothe generator in the remelting station and a fixed assembly comprisingan inductive plasma torch and a 150 kHz generator. The torch is poweredby a gas mix composed of 40% HCl and 60% argon. The treatment durationwas 1 h per batch.

Each plasma treated silicon batch was then solidified and segregated inan 0.67 m² ingot mould provided with a pouring spout and placed in areverberatory furnace electrically heated by graphite bars acting asresistances, heat losses from the furnace taking place mainly throughthe hearth. The power of the furnace was held at 45 kW. The liquidus waspoured after 3 hours waiting. The poured mass resulted in a 36 kg ingot,while the mass of recovered solidified silicon was 162 kg.

Liquid sampling was done on the raw plasma treated silicon and theanalysis results were as follows:

-   -   Iron=280 ppm; Calcium=23 ppm; Aluminium=14 ppm; Titanium=9 ppm;        Boron=3 ppm; Phosphorus=8 ppm; Carbon=50 ppm; Oxygen=80 ppm.

The results of the analysis on the solid silicon obtained aftersegregation after plasma refining were as follows:

-   -   Iron=160 ppm; Calcium=9 ppm; Aluminium=8 ppm; Titanium=5 ppm;        Boron=3 ppm; Phosphorus=8 ppm; Carbon=50 ppm; Oxygen=90 ppm.

Example 2

The test in example 1 was repeated, attempting to get the best possiblequality of the final silicon from this process.

A 2 t ladle of liquid silicon was sampled from the production of siliconfrom a submerged arc electric furnace and a chlorine refining treatmentwas carried out on it; the results of the analysis on the sampled liquidsilicon in the ladle were as follows:

-   -   Iron=0.25%; Calcium=90 ppm; Aluminium=210 ppm; Titanium=240 ppm;        Boron=32 ppm; Phosphorus=20 ppm; Carbon=100 ppm; Oxygen=400 ppm.

This ladle was then treated at low pressure for 30 minutes with argoninjection into the bottom of the ladle through a porous brick; the argonflow was 0.7 Nm³/hour.

Some of the liquid silicon thus obtained was poured into a sinteredsilica ingot mould provided with a pouring spout with a capacity of 600kg of silicon; this ingot mould with an area of 2 m² was placed in areverberatory furnace electrically heated using graphite bars acting asresistances, heat losses from the furnace taking place mainly throughthe hearth. The furnace power was adjusted to 50 kW so that about 50% ofthe silicon was solidified in the ingot mould in about 2.5 h.

After 150 minutes waiting, the liquid remaining in the ingot mould waspoured through the spout and produced a 290 kg ingot. The mass of thesolid silicon remaining in the ingot mould was 295 kg and the result ofthe analysis was:

-   -   Iron=95 ppm; Calcium=23 ppm; Aluminium=12 ppm; Titanium=9 ppm;        Boron=32 ppm; Phosphorus=6 ppm; Carbon=100 ppm; Oxygen=400 ppm.

The operation was repeated several times in order to obtain a sufficientquantity of silicon of this quality.

The silicon obtained was remelted in 200 kg batches under an argonatmosphere in a 250 kW induction furnace operating at 1000 Hz, beginningwith a metallurgical liquid silicon heel; production from the firstthree batches was discarded to be sure that the furnace was correctlyflushed.

The production from the next batches was transferred batch by batch forrefining treatment under the same conditions as in example 1. Thetreatment duration was 2 h per batch.

Each plasma treated silicon batch was then subjected to a segregatedsolidification in an 0.67 m² ingot mould equipped with a pouring spoutand placed in a reverberatory furnace electrically heated by graphitebars acting as resistances, heat losses from the furnace taking placemainly through the hearth. The power of the furnace was held at 55 kW.The liquidus was poured after 6 hours waiting. The poured mass resultedin a 30 kg ingot, while the mass of recovered solidified silicon was 164kg.

Liquid sampling was done on the raw plasma treated silicon and theanalysis results were as follows:

-   -   Iron=85 ppm; Calcium=23 ppm; Aluminium=12 ppm; Titanium=9 ppm;        Boron=2 ppm; Phosphorus=5 ppm; Carbon=30 ppm; Oxygen=50 ppm.

The results of the analysis on the solid silicon obtained aftersegregation after plasma refining were as follows:

-   -   Iron=16 ppm; Calcium=9 ppm; Aluminium=7 ppm; Titanium=4 ppm;        Boron=2 ppm; Phosphorus=5 ppm; Carbon=30 ppm; Oxygen=50 ppm.

1. Method for making a photovoltaic quality silicon from an oxygen orchlorine refined metallurgical silicon containing less than 500 ppm ofmetal elements and comprising: remelting of the refined silicon, under aneutral atmosphere, in an electric furnace equipped with a hot crucible,transfer of the remelted silicon for plasma refining, in an electricfurnace equipped with a hot crucible, plasma refining of the moltensilicon with a mixture of argon and at least one gas from the groupconsisting of chlorine, fluorine, hydrochloric acid and hydrofluoricacid, as plasma-forming gas, the mix containing from 5 to 90% of argon,casting under controlled atmosphere in an ingot mould, in whichsegregated solidification takes place.
 2. Method according to claim 1,characterised in that the silicon is prepared with less than 500 ppm ofmetallic elements, using a segregated solidification operation toconcentrate the metallic impurities in the liquid fraction.
 3. Methodaccording to claim 1, characterised in that remelting is done onsuccessive batches.
 4. Method according to claim 1, characterised inthat remelting and plasma refining of silicon are done in two differentworkstations.
 5. Method according to claim 1, characterised in thatsilicon is transferred between the remelting operation and the plasmarefining operation by displacement of an assembly composed of the casingof the furnace, the induction coil, the crucible, and liquid silicon. 6.Method according to claim 1, characterised in that plasma refining isdone using an HF-argon and/or HCl-argon gas mix containing between 50%and 70% argon.
 7. Method according to claim 1, characterised in that theplasma source is an inductive torch powered by an electricity source ata frequency of between 100 kHz and 4 MHz.
 8. Method according to claim2, characterised in that the first segregated solidification, beforeplasma refining, is controlled so that the solidification front advancevelocity is below 2×10⁻⁵ m/s.
 9. Method according to claim 1,characterised in that the segregated solidification, after plasmarefining, is controlled so that the solidification front advancevelocity is below 10⁻⁵ m/s.
 10. Method according to claim 9,characterised in that the solidification front advance velocity is below5×10⁻⁶ M/s.
 11. Method according to claim 1, characterised in thatsegregated solidification operations take place in a reverberatoryfurnace.
 12. Method according to claim 1, characterised in that electricfurnaces used for silicon remelting and plasma refining operations areinduction furnaces.
 13. Method according to claim 1, characterised inthat electric furnace crucibles used for silicon remelting and plasmarefining operations are made either of silica, carbon, graphite, orsilicon carbide.
 14. Silicon designed in particular for making solarcells, with a total content of impurities ranging between 100 and 400ppm, a boron content ranging between 0.5 and 3 ppm, and aphosphorus/boron ratio ranging between 1 and 3, and a content ofmetallic elements between 30 and 300 ppm.
 15. Silicon according to claim14, characterised by a total content of impurities ranging between 100and 250 ppm, a boron content ranging between 0.5 and 2 ppm, and acontent of metallic elements between 30 and 150 ppm.
 16. Siliconaccording to claim 14, characterised by an iron content ranging between10 and 20 ppm.
 17. Method according to claim 2, characterised in thatremelting is done on successive batches.
 18. Method according to claim2, characterised in that remelting and plasma refining of silicon aredone in two different workstations.
 19. Method according to claim 3,characterised in that remelting and plasma refining of silicon are donein two different workstations.
 20. Silicon according to claim 15,characterised by an iron content ranging between 10 and 20 ppm.